1. Field of the Invention
This invention relates to a method and apparatus for non-linear quantization of a signal using an analog to digital converter (ADC), and more particularly, to a method and apparatus allowing finer quantization in a particular part of the dynamic range of the signal without having to increase the number of bits produced by the ADC and, so its total dynamic range.
2. Description of the Related Art
Sometimes it is desirable to digitize a signal in such a way as to have more quantizing levels in one range of signal values than in another. For example, in digital video processing, the effects of quantization noise are distinguishable more readily at black than at white. While adding more bits to the ADC does decrease the quantization noise at black, it also adds more bits and more quantization resolution to the signal at white, where the benefits may not be so distinguishable or desirable. Adding more bits to the ADC adds additional cost and increases the size of the ADC. For example, using an 8-bit system, there are 256 levels available for quantization. Using a 10-bit converter, finer quantization of signals in a range of values around the black level would be available, as there would be 1,024 levels. High speed ADCs capable of operating at these levels, however, may not be available or may be cost prohibitive.
U.S Pat. No. 3,452,297 to Kelly et al (hereinafter Kelly) teaches a non-linear encoder for converting an analog signal into a corresponding binary pulse code signal. The quantizing characteristic of Kelly is defined by major quantizing levels, each pair of major quantizing levels having seven minor quantizing levels between them. The major levels are generated first using a first set of logically controlled current generators and a plurality of corresponding impedances to obtain the major level which is nearest to but less than the input signal. Upon determining the correct major level, the correct minor level is similarly determined using a second series of logically controlled current generators which employ the impedance corresponding to the correct first generator so that the minor levels are added to the correct major level.
U.S. Pat. No. 3,305,855 to Kaneko describes a converting analog to digital converter which quantizes, for example, voice signals with minor quantization steps for signals of smaller amplitudes as compared with quantization steps for signals of larger amplitudes. The quantization step size used for any analog level is determined from an N.sup.th order polynomial where N is the number of bits in the digital output signal. The apparatus of Kaneko results in a non-linear transfer characteristic by using an ADC having an integral pulse-code-modulation encoder.